Intercellular communication is essential for normal cardiac impulse propagation. It is thought to be mediated by gap junction channels. Three gap junction proteins, known as connexins (Cxs), are expressed in the heart; these are Cx40, Cx43 and Cx45. However, the specific role of the individual connexins in normal and abnormal propagation is unknown. The recent development of connexin knockout mice now makes such knowledge possible. Our overall objective is to investigate the electrophysiological consequences of the absence of specific gap junction channels on impulse propagation in mice lacking either Cx43 or Cx40. We will combine electrocardiographic, microelectrode and patch- clamping techniques, with high-resolution microscopic and macroscopic optical recordings of potentiometric dye fluorescence, to measure relevant electrophysiological parameters, which may be affected by Cx43 and Cx40 null mutation. Our Specific Aims are 1. To determine the electrophysiological consequences and pro-arrhythmic effects of reduced intercellular coupling in the ventricles of neonatal homozygote and heterozygote Cx43 knockout mice. We hypothesize that the lack of Cx43 results in a reduction of intercellular communication leading to discontinuous conduction, with an increase in the variability of local conduction times during both sinus rhythm and pacing. Moreover, we surmise that impulse blockade in the homozygote mice will occur preferentially in the direction transverse to fiber orientation and at much slower frequencies than in wildtype or heterozygous mice. 2. To determine the role of Cx40 in impulse propagation in the specialized conducting system, and ventricles of the adult mouse heart. Our hypothesis here is that the null mutation of Cx40 decreases intercellular communication in the specialized conduction system and slows Purkinje fiber conduction velocity, leading to an apparent bundle branch block configuration in the electrocardiogram, and facilitating the induction of reentrant arrhythmias. Further, because of source-sink relationships, we expect that conduction block is more likely to occur at branch points and the Purkinje-muscle than along the Purkinje bundles. 3. To determine the effects of Cx40 null mutation on sinus rhythm and impulse propagation in the atria of the adult mouse heart. Cx40 is expressed in both atria and the sinus node of the mouse. It has been shown that the lack of Cx40 results in P wave prolongation and atrial tachyarrhythmias, including fibrillation. However, in the absence of such arrhythmias, RR interval is unaffected in Cx40-/- mice. We postulate that Cx40 plays a crucial role in intercellular communication in the atria and helps maintain normal intraatrial conduction, but is not essential for synchronized pacemaker discharge in the sinus node. We propose also that the reduction in intercellular communication within the atria of the CX40 knockout mouse is accompanied by an increased susceptibility to reentrant arrhythmias. Overall, the studies proposed are highly significant in that they will provide definite proof or refutation to long-held assumptions regarding the fundamental role of connexins in cardiac electrophysiology and arrhythmias.